It is already known to anchor permanently a dental prosthesis in the jaw with the aid of screws made of titanium implanted in the jawbone. The screws are anchored in holes in the bone so that the upper part of the screw is situated on a level with or immediately below the upper surface of the jaw bone. The screw is then covered over with a mucous membrane flap and is left unstressed for a rest period of 3-6 months in order that the bone grows onto and form a unit with the implanted screw. After the rest period, the screw is uncovered and a distance element, also preferably made of titanium, is arranged on the screw, whereupon a dental prosthesis is anchored on the distance element.
As a result of the high oral stresses during biting and chewing, dental prostheses have in general been anchored by means of a bridge construction with the aid of a number of fixtures, for example six pieces. If any of the screws comes loose, those remaining then ensure that the secure anchoring is still maintained.
In recent years, however, efforts have been made to provide secure anchoring of individual teeth. In the case of such a single-tooth replacement, one single screw is to be able to absorb all arising oral stresses such as torsion, stretching and pressure forces. Especially important in this respect is the torsional load which tends to loosen the screw joint between distance screw and screw (fixture). For single-tooth replacements which are subjected the high oral stresses, it is of course important that the design and anchoring of the screw and the distance element are the best possible in order to prevent the screw joint from being loosened. In the Swedish Patent 87 01949-3, a screw joint anchoring is described which affords an increased anchoring stability compared with previously known dental prostheses of this type. The screw joint anchoring is designed in such a manner that the dental prosthesis is unlikely loosen and has great capacity for transmitting stresses.
It is, however, not only important for the design of the screw and the distance element to be the best possible. The operational technique and the mounting of screw and distance element are also to be optimal. As a result of the torsional load, it is important for the distance screw to be tightened firmly. In this connection, the entire tightening torque comes to stress the screw which then risks loosening if special measures are not taken. According to the abovementioned Swedish Patent 87.01949-3, the distance element is provided with an inward, internal holder, in which fits one part of a double screwdriver adapted for the purpose fits. The other part of the double screwdriver is designed as a conventional screwdriver and fits the screwdriver slot in the head of the distance screw.
The distance element is arranged on the firmly rooted screw (the fixture) in such a manner that first the spacer is locked firmly against the fixture with the aid of the distance screw which is screwed down into a bore in the fixture with the aid of the double screwdriver, in doing which it is ensured that the legs of the tubular part engage with recesses in the spacer and the other part of the screwdriver is passed through the tubular part so that it engages in the screwdriver slot in the screw head of the distance screw. Upon mounting, it is endeavoured to apply to the screwdriver parts torques of the same size but in opposite directions. This can, however, be difficult to achieve in practice, especially in the case of high tightening torques.
The double screwdriver described above is of course applicable only in the case of manual tightening of the screw joint. There is, however, a desire to be able to carry out the tightening mechanically, that is to say with the drilling equipment available at the time of the dental operation. This facilitates the mounting for the dentist and also has the advantage that the tightening torque can be controlled.